The present invention relates to signal communications and, in particular, to reception of encoded and modulated signals over a communications channel.
One type of communications channel which is expanding particularly rapidly is wireless communications, particularly as more radio spectrum becomes available for commercial use and as cellular phones become more commonplace. In addition, analog wireless communications are gradually being supplemented and even replaced by digital communications. In digital voice communications, speech is typically represented by a series of bits which may be modulated and transmitted from a base station of a cellular communications network to a mobile terminal device such as a cellular phone. The phone may demodulate the received waveform to recover the bits, which are then converted back into speech. In addition to voice communications, there is also a growing demand for data services, such as e-mail and Internet access, which typically utilize digital communications.
There are many types of digital communications systems. Traditionally, frequency-division-multiple-access (FDMA) is used to divide the spectrum up into a plurality of radio channels corresponding to different carrier frequencies. These carriers may be further divided into time slots, generally referred to as time-division-multiple-access (TDMA), as is done, for example, in the digital advanced mobile phone service (D-AMPS) and the global system for mobile communication(GSM) standard digital cellular systems. Alternatively, if the radio channel is wide enough, multiple users can use the same channel using spread spectrum techniques and code-division-multiple-access (CDMA).
A typical digital communications system 19 is shown in FIG. 1. Digital symbols are provided to the transmitter 20, which maps the symbols into a representation appropriate for the transmission medium or channel (e.g. radio channel) and couples the signal to the transmission medium via antenna 22. The transmitted signal passes through the channel 24 and is received at the antenna 26. The received signal is passed to the receiver 28. The receiver 28 includes a radio processor 30, a baseband signal processor 32, and a post processing unit 34.
The radio processor typically tunes to the desired band and desired carrier frequency, then amplifies, mixes, and filters the signal to a baseband. At some point the signal may be sampled and quantized, ultimately providing a sequence of baseband received samples. As the original radio signal generally has in-phase (I) and quadrature (Q) components, the baseband samples typically have I and Q components, giving rise to complex, baseband samples.
The baseband processor 32 may be used to detect the digital symbols that were transmitted. It may produce soft information as well, which gives information regarding the likelihood of the detected symbol values. The post processing unit 34 typically performs functions that depend on the particular communications application. For example, it may convert digital symbols into speech using a speech decoder.
A typical transmitter is shown in FIG. 2. Information bits, which may represent speech, images, video, text, or other content material, are provided to forward-error-correction (FEC) encoder 40, which encodes some or all of the information bits using, for example, a convolutional encoder. The FEC encoder 40 produces coded bits, which are provided to an interleaver 42, which reorders the bits to provide interleaved bits. These interleaved bits are provided to a modulator 44, which applies an appropriate modulation for transmission. The interleaver 42 may perform any type of interleaving. One example is block interleaving, which is illustrated in FIG. 3. Conceptually, bits are written into rows of a table, then read out by column. FIG. 3 shows an example of 100 bits, written into a 10xc3x9710 table.
Another example of interleaving is diagonal interleaving, in which data from different frames are interleaved together. Diagonal interleaving is illustrated in FIG. 4. Each frame is block interleaved using block interleavers 50a, 50b, and 50c. Using switches 52a, 52b, and 52c, interleaved bits from each frame are split into two groups. The multiplexors 54a and 54b combine groups of bits from different frames to form transmit frames. In TDMA systems, different transmit frames generally would be sent in different time slots.
The modulator 44 may apply any of a variety of modulations. Higher-order modulations, such as those illustrated in FIGS. 5A and 5B, are frequently utilized. One example is 8-PSK (eight phase shift keying), in which 3 bits are sent using one of 8 constellation points in the in-phase (I)/quadrature (Q) (or complex) plane. In FIG. 5A, 8-PSK with Gray coding is shown in which adjacent symbols differ by only one bit. Another example is 16-QAM (sixteen quadrature amplitude modulation), in which 4 bits are sent at the same time as illustrated in FIG. 5B. Higher-order modulation may be used with conventional, narrowband transmission as well as with spread-spectrum transmission.
A conventional baseband processor is shown in FIG. 6. A baseband received signal is provided to the demodulator and soft information generator 60 which produces soft bit values. These soft bit values are provided to the soft information de-interleaver 62 which reorders the soft bit values to provide de-interleaved soft bits. These de-interleaved soft bits are provided to the FEC decoder 64 which performs, for example, convolutional decoding, to produce detected information bits.
A second example of a conventional baseband processor is shown in FIG. 7. This processor employs multipass equalization, in which results, after decoding has completed, are passed back to the equalization circuit to re-equalize, and possibly re-decode, the received signal. Such a system is described, for example, in U.S. Pat. No. 5,673,291 to Dent et al. entitled xe2x80x9cSimultaneous demodulation and decoding of a digitally modulated radio signal using known symbolsxe2x80x9d which is hereby incorporated herein by reference. For the circuit illustrated in FIG. 7, the processor typically initially performs conventional equalization and decoding. After decoding, the detected information bits are re-encoded in the re-encoder 74 and then re-interleaved in the re-interleaver 72 to provide information to the multipass equalizer and soft information generator 70 which re-equalizes the received baseband signal using the detected bit values. Typically, because of diagonal interleaving or the fact that some bits are not convolutionally encoded, the second pass effectively uses error corrected bits, as determined and corrected in the first pass, to help detection of other bits, such as bits which were not error correction encoded.
Both single pass and multipass baseband processors as described above typically use conventional forward error correction (FEC) decoders. Conventional FEC decoders typically treat each soft bit value as if it were independent of all other values. For example, in a Viterbi decoder for convolutional codes, soft bit values are generally correlated to hypothetical code bit values and added. As the soft bit values typically correspond to loglikelihood values, adding soft values corresponds to adding loglikelihoods or multiplying probabilities. As the Viterbi decoder corresponds to maximum likelihood sequence estimation (MLSE) decoding, multiplying probabilities generally assumes that the noise on each bit value is independent.
For lower-order modulation, with Nyquist pulse shaping and nondispersive channels, independent noise is often a reasonable assumption. For example, for quadrature phase shift keyed (QPSK) modulation, one bit is generally sent on the I component and a second bit is sent on the Q component. Because noise is typically uncorrelated between the I and Q components, the noise on these two bits would generally be independent. However, with higher-order modulation, noise values on the different bits are generally not independent. Consider the 8-PSK example shown in FIG. 5A. As 3 bits are affected by only 2 independent noise values (I and Q noise components), the noise on the 3 soft bit values is expected to be correlated. Thus, with higher-order modulation, the conventional approaches to demodulation and decoding ignore the fact that bit soft values may be related through correlated noise. As a result, performance may be reduced.
In a more general sense, bit likelihoods may be coupled in many ways. For 8-PSK with Nyquist pulse shaping, groups of 3 bits are generally coupled by the modulation. With partial response pulse shaping, overlapping groups of bits may be coupled through the pulse shape. Differential modulation may also couple successive symbols. Bit coupling may also be introduced by the communication channel, for example, bit coupling may result from multipath time dispersion, in which symbols overlap with one another, for example, due to signal echoes.
In embodiments of the present invention, methods for decoding a signal received over a channel from a transmitter are provided. The methods include decoding a received signal having a first bit and a second bit based on a coupling of the first bit to the second bit introduced by at least one of the channel or the transmitter.
In other embodiments of the present invention, baseband processors are provided for decoding a signal received over a channel from a transmitter. The baseband processor in various embodiments includes a demodulator that demodulates the received signal to provide demodulator soft information associated with a plurality of bits in the received signal. An intermediate value memory configured to store the demodulator soft information associated with a plurality of bits in the received signal is also provided. The baseband processor further includes a soft information generator circuit coupled to the intermediate value memory. A decoder is provided configured to process soft information associated with the plurality of bits in the received signal from the soft information generator circuit to provide decoder soft information associated with ones of the plurality of bits in the received signal. The soft information generator circuit is configured to generate the soft information associated with the plurality of bits in the received signal based on a coupling of one of the plurality of bits to another of the plurality of bits introduced by at least one of the channel or the transmitter and responsive to the demodulator soft information associated with the plurality of bits.
As will further be appreciated by those of skill in the art, the present invention may be embodied as methods, systems and/or baseband processors.